A charge retention medium is an insulator (dielectric) which maintains a certain capacity of charge on its surface or in its interior. The charge retention medium is classified into two types, i.e. one which maintains homocharge (independent charge) in its interior by injecting electric charge to an insulator (dielectric), and one such that an insulator (dielectric) itself is polarized to maintain a certain capacity of charge as the entire material.
As the charge retention medium which maintains homocharge, a material having high insulation properties and a low water-absorbing property is used. Specifically, such a material may be one having positive or negative charge injected by a method such as corona discharge to a hydrocarbon organic polymer material such as polypropylene, polystyrene or a cycloolefin copolymer; a fluorinated polymer material such as polytetrafluoroethylene (PTFE) or a tetrafluoroethylene/hexafluoropropylene copolymer (FEP); an inorganic insulating material formed by thermal oxidation or plasma CVD, such as SiO2; or the like. The charge retention medium having the homocharge maintained is applicable, utilizing the potential appearing on its surface, to an electrostatic induction conversion device (such as a power generation device, a microphone or a speaker) or to a filter having an increased dust collection efficiency. Further, it is also applicable to a piezoelectric device or an actuator since it has opposite polarity as between on the front surface and on the rear surface and thus it has a property as if it is polarized as the entire medium.
As the charge retention medium such that the insulator (dielectric) itself is polarized, a material having a high anisotropy (large dipole moment) in its molecule is used. A material having a ferroelectric property is mainly used, and specifically, an anisotropic organic polymer material such as polyvinylidene fluoride or a liquid crystalline polymer; an anisotropic inorganic material such as PZT (Pb(Zr,Ti)O3); or the like may be mentioned. The charge retention medium of a type such that the insulator (dielectric) itself is polarized is applicable to a recording medium such as a memory utilizing its strong dielectric property, a piezoelectric device or an actuator utilizing the polarization of the entire medium, or the like.
Such a charge retention medium is used, specifically, for the following applications.
The applications of one which maintains the homocharge may, for example, be an electret to be used for an electrostatic induction conversion device (such as a power generation device, a microphone, a speaker, an actuator or a sensor) which converts an electric energy and a kinetic energy; an electrostatic charge retention layer in an electrostatic charge recording medium having an electrostatic charge retention layer in an electrode layer (Patent Document 1); a surface member of a cleaning roller which cleans away a toner remaining on the surface of a photoreceptor in an image forming apparatus (such as a copying machine or a printer) (Patent Document 2); a member for particles for image display having a color and an electrostatic property to be used for an image display device such as electronic paper (Patent Document 3); a piezoelectric electret film which, in a printing machine in which an inking roller is pressed against a printing plate, measures the pressing of the inking roller against the printing plate (Patent Document 4); and a dust collection filter.
The applications of one such that an insulator (dielectric) itself is polarized may, for example, be an electrostatic charge retention layer in an electrostatic charge recording medium having an electrostatic charge retention layer on an electrode layer (Patent Document 1); a surface member of a cleaning roller which cleans away a toner remaining on the surface of a photoreceptor in an image forming apparatus (such as a copying machine or a printer) (Patent Document 2); and a piezoelectric electret film which, in a printing machine in which an inking roller is pressed against a printing plate, measures the pressing of the inking roller against the printing plate (Patent Document 4).
Particularly, an electrostatic induction conversion device (such as a power generation device or a microphone) having an electret as the charge retention medium provided on the surface of a substrate attracts attention. As the electret, for example, Patent Document 5 proposes an electret having electric charge injected to a precursor (a coating film or a film) of an electret obtainable from a composition containing a compound (A) having a molecular weight of from 50 to 2,000 having at least two polar functional groups and a polymer (B) having a number average molecular weight exceeding 2,000, having reactive functional groups reactive with the polar functional groups. Further, Patent Document 5 discloses, as the polymer (B), a fluorinated polymer (e.g. a fluorinated polymer having a cyclic structure in its main chain, or a fluorinated copolymer (hereinafter sometimes referred to as ETFE) having repeating units based on ethylene and repeating units based on tetrafluoroethylene).
As the material for the charge retention medium, in the case of using a fluorinated polymer, ETFE may sometimes be used in view of the following points.                Since ETFE is inexpensive as compared with a fluorinated polymer having a cyclic structure in its main chain, the charge retention medium can be obtained at a low Cost.        Since ETFE has crystallinity, dispersion at the nano-order level is possible by introducing an additive to the charge retention medium so that it is incorporated in the amorphous portion between crystals.        Since ETFE has crystallinity, it is less likely to be softened even at the glass transition point or higher.        
Further, Patent Documents 6 and 7 disclose that ETFE is soluble in a specific organic solvent (a fluorinated aromatic compound or a linear hydrocarbon compound having a carbonyl group) under heating at a temperature of at most the melting point of ETFE, and Patent Document 6 discloses that when an ETFE solution having ETFE dissolved in a fluorinated aromatic compound is cooled, an ETFE dispersion is obtained.